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Processes and Threads

“The OS illusion: making one CPU act like it’s 20.”

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Big Idea

Every running program isn’t just code — it’s a process,
an isolated world with its own memory, resources, and identity.

The OS juggles hundreds of these, switching between them so smoothly you think it’s multitasking.

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Process: The Living Program

A process is a running instance of a program with:

• Its own memory space
• CPU time slices
• Open files / I O handles
• Unique process ID (PID)

Analogy:
A person at a desk, working with their own supplies, taking turns at the CPU coffee machine.

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Process Lifecycle

States

• New – being created
• Ready – waiting for CPU
• Running – currently executing
• Waiting – blocked for I/O or event
• Terminated – finished or killed

Transitions: scheduler decisions, I/O events, system calls.

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Visual Model

New → Ready → Running → Terminated
 Waiting 

graph TD;
    A-->B;
    A-->C;
    B-->D;
    C-->D;

Each arrow represents a system call or interrupt that changes who holds the CPU.

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Threads: The Mini-Processes

A thread is a lightweight process inside another process.

All threads share:
• Memory space
• Code / data sections
• File descriptors

Each thread has:
• Own stack
• Own program counter
• Own schedule

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Process vs Thread Comparison

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Why Threads Exist

• To do multiple tasks within one program (browser tabs)
• To overlap computation with I/O
• To scale across multiple cores
• To confuse CS students since 1970

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Context Switching

When the CPU switches threads/processes:

1. Save current CPU state (registers, PC, etc.)
2. Load next thread’s state
3. Resume execution

Overhead: switching too often wastes cycles — scheduling aims to minimize that.

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Scheduling Overview (Concept Only)

Schedulers decide:

• Who runs next
• For how long (quantum)
• When to pre-empt

Common strategies:

• FCFS – First Come, First Served
• RR – Round Robin
• SJF – Shortest Job First

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System Calls — The OS Entry Points

Processes interact with the OS via system calls:
fork(), exec(), wait(), read(), write() …

They’re the controlled bridge from user mode → kernel mode.

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Process Creation (UNIX Style)

pid = fork()
if pid == 0:
    exec("child_program")
else:
    wait(pid)

• fork() duplicates the process
• exec() replaces it with new code
• wait() synchronizes parent and child

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Visualization Idea – “Process Zoo”

Simulate multiple processes competing for CPU:

• Random burst and I/O wait times
• Log state transitions
• Scheduler: Round Robin for fairness

Shows the lifecycle in motion instead of static theory.

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Why It Matters

Understanding processes and threads explains:

• Multitasking
• Deadlocks (next week)
• Virtual memory
• Containers and virtualization

Every higher-level abstraction depends on these primitives.

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Exit Prompt

Finish the sentence:

“A thread is like a __ because it __.”

Examples

• “ …like a roommate — shares space but has its own schedule.”
• “ …like a worker bee — independent but part of one hive.”

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Summary

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Next Topic

Next Week 01 Topic 03: Scheduling & Context Switching

References & Credits

topic: "Operating Systems Week 1 Lecture Slides"
focus: "Processes and Threads"
format: "Markdown-based slide content"
author: "T. Griffin and OpenAI GPT-5"
credit: "Concept scaffolding with ChatGPT (OpenAI GPT-5)"